A manufacturing apparatus which manufactures a film such as a multilayer polyimide film uses an extrusion die which continuously extrudes a liquid resin film such as polyamide acid, which is formed into a film, in a predetermined width and a predetermined thickness. In the extrusion die, a seal part (packing) may be provided in a space between (i) an end in a width direction of a lip opening, the width direction corresponding to a width direction of the film (hereinafter referred to as a “lip edge”) and (ii) an end plate so as to prevent a leakage of a liquid resin.
However, there may appear a gap or a level difference (groove) between the lip edge and the end plate due to a small variation (individual difference) in size, thickness, and shape of the seal part, and an error occurring while the seal part is being provided. In other words, the lip edge, the seal part, and the end plate cannot be flush with each other. When a gap or a level difference appears in a space between the lip edge and the end plate, a pool of a liquid resin is easily produced in the space. This makes a flow of the liquid resin unstable and thus causes each end of the liquid resin film to be unstable in shape and thickness. The liquid resin thus having pooled cures to be agglomerate, and the agglomerate may adhere as a foreign matter to the liquid resin film to be extruded from the lip opening.
In a case where a film is manufactured while a gap or a level difference remains between the lip edge and the end plate, it is difficult to stably and continuously produce a film, which may deteriorate in quality. This requires a method for making the each end of the liquid resin film stable in shape and thickness.
There have been proposed various techniques for changing an ejection width of a resin film, which is to be a thermoplastic resin film, during continuous production by providing, at a lip edge of an extrusion die, an ejection width changing section such as an outer deckle so that the ejection width changing section covers a part of the lip edge (see Patent Literatures 1 through 8).